Tutorial #109
Peter Dijkstra, George Riley
December 2010
Description
Today’s thinner assemblies and smaller feature sizes are more challenging to plasma clean and more vulnerable to plasma cleaning damage. A new plasma advance allows faster, more pervasive plasma cleaning and surface treatments with a much lowered risk of damage.
Plasmas clean through both physical and chemical processes. Physical cleaning uses energetic ultraviolet photons and charged particles to forcefully break up contamination molecules. However, these may also breakup molecules of sensitive device layers, altering device performance or reliability
Physical cleaning may also raise device temperatures to harmful levels, or cause charge buildup and electrical arcing. Physical cleaning is slow to penetrate small gaps, and does not clean large areas uniformly, making it unsuitable for cost-reducing batch processes.
Chemical cleaning uses highly reactive, electrically neutral free radicals to remove contaminants. The radicals alter surface energies and convert contaminant molecules to simpler, lower vapor pressure compounds, easily removed as gases.
A recently-patented plasma improvement, High Density Radical Flux (HDRF®) technology, segregates the damaging physical cleaning flow from the free radicals, while creating free radical concentrations that are 500 to 1,000 times higher than in conventional plasmas.
Inductive coupling of energy to the plasma, combined with a proprietary plasma confinement technology, gives 100% gas dissociation. Figure 1 shows the increase in the concentration of oxygen radicals O* produced as oxygen flow increases.
Figure 1. Oxygen radical variation with oxygen flow rate. (courtesy Nanoplas)
High concentrations of reactive radicals with no charged particles, unwanted photons, or high temperatures allow the gases to modify surface characteristics, including surface energy and chemical composition, without physical damage. This new capability is unique to the HDRF process.
HDRF eliminates arcing and heating, avoiding thermal damage while allowing low or even room temperature operation. The high concentration of charge-free radicals acts uniformly in all directions, penetrating small cavities as well as batch-loaded magazines of substrates and wafers to speed up production.
Applications
- Underfill Faster wicking speed, increasing throughput; higher and more uniform underfill fillets for stronger bonding and balanced mechanical forces.
- Cleaning Penetrates chip-to-substrate gaps smaller than 30 µm; allows cleaning and activating after solder reflow to eliminate flux residue before underfilling.
- Protection Surface treatment before underfill improves bond strength, wicking time, fillet uniformity, and drop-test survival rates.
- Die Attach Activation and roughened surface strengthen bond and increases thermal conductivity.
- Wire Bonding Removes contamination and oxidation; increased bond strength, improved reliability, and higher yield.
- Copper Bonnding Oxide removal crucial for replacing gold with copper wire bonding.
- Encapsulation Surface treatment before underfill improves bond strength, wicking time, fillet uniformity, and drop-test survival rates.
- Production Uniformly handles multiple magazine-loads of chips or wafers simultaneously.
- Versatility Flexible, expandable, and multi-purpose; wide range of pressures and chemistries; multistep processing , sequential or simultaneous multi-gas exposure. Alternative operating modes for removing heavy residues, or stripping BCB film.
FOR MORE INFORMATION:
Dijkstra, P. and Riley, G. “Damage-Free Plasma Surface Treatments,” Wafer and Device Packaging and Interconnect, Volume 1, #7, November / December 2010, pp 28 – 31.